Ninja 250 is a great starter bike, and your dad made a good choice starting you on that. Every new rider wants a gixxer, but remember, gixxers are for squids
As far as going 80-90 at high RPM, if you're in 6th gear then there really isnt a way to lower those rpm (other than getting a smaller rear sprocket, which i don't recommend).
I'd also advise you to get at least a couple thousand miles under your belt before riding two-up.